Planar Lightwave Circuit (PLC) technology has become the dominant technology platform for the integration of existing and new optical-assembly functions into chip-based silica-waveguide integrated optical circuits. See, for example, K. Okamoto (2010), “Fundamentals of Optical Waveguides” (Academic Press), incorporated herein by reference for basics of optical waveguide technology and formation of planar lightwave circuits. Y-Branch waveguides are an important integrated-optic circuit element in a broad range of optical circuits. Y-Branch waveguides are used to distribute the optical signal from a single waveguide into two separate waveguides (a ‘splitter’); or to recombine two optical signals (according to interference principles) that have propagated along separate paths (a ‘combiner’). This 1×2 nature of the Y-Branch waveguide is expanded to greater splitting or combining ratios by integrated cascading of Y-Branch circuit elements.
Optical waveguides do not ‘contain’ the propagation of optical signals as wires contain the propagation of electrical signals. Waveguides merely influence the propagation of optical signals along or nearby their paths. Imperfections in the waveguide structure generally result in some of the light of the optical signal merely propagating away from the waveguides. This results in a reduction of energy in the guided optical signals and diminished signal integrity. It is therefore a significant design objective of such waveguide circuit elements that they have as little excess loss as possible. Since Y-Branch elements are often cascaded to generate larger manifolds, even a small percentage of loss in each Y-Branch can lead to undesirable circuit loss.
Optical networks generally comprise transmission components that are designed to transmit bands of wavelengths over reasonable distances. The bands of wavelengths generally comprise signals intended for a plurality of customers/users. Thus, a single optical fiber can be used to simultaneously transmit a plurality of signals that are subsequently divided for delivery. Similarly, individual signals are combined for transmission over common lines prior to eventual division for routing and/or delivery. Individual bands thus are divided into smaller wavelength ranges corresponding to signals relating to individual users, including aggregations of a few users, and multiplexing and de-multiplexing functions can be used to convert between combined signals for common lines and individual signals for routing and/or interfacing with individual users. Planar lightwave circuits are generally used effectively for many optical functions used for wavelength division multiplexing.